The present invention relates to a pressure sensor, and more particularly to a sensor electrode coated with an insulating body on a base plate, in which the dispersion in a coating thickness of the insulating body is decreased.
Referring to FIGS. 3 to 6, a related sensor of this type will be described below. In FIG. 3, a switch device 1 includes a pair of first electrodes 3 mounted on a base plate 2, and a C-shaped second electrode 4 which is circumferentially arranged around the first electrodes 3 so as to be outwardly apart therefrom as shown in FIG. 4. The first electrodes 3 and the second electrode 4 are covered with a resist 5, and a pair of carbon electrodes 6 are provided on an upper face of the resist 5 and above the first electrodes 3.
Moreover, there is provided a click rubber 7 above and opposed to the first electrodes 3 and the second electrode 4. The click rubber 7 has a substantially cylindrical body 8 and a flexible leg 9 extending downwardly from an outer peripheral part of the cylindrical body 8 at a lower end thereof. In addition, a conductive rubber 10 substantially in a disc-like shape is provided in a center part of the lower end of the cylindrical body 8. The conductive rubber 10, the first electrodes 3 and the second electrode 4 constitute an electrostatic capacitance sensor 13.
Further, a button 11 is mounted on an upper face of the click rubber 7. An upper cover 12 for covering an upper part of the click rubber 7, the first electrodes 3 and the second electrode 4 is provided to press down at a bottom parts 9a of the flexible leg 9 of the click rubber 7, so that an upper part of the button 11 is projected through an opening 12a which is formed in an upper part of the upper cover 12.
FIG. 5 shows a sensor circuit 14 of the switch device 1. The sensor circuit 14 includes a resistor 16 for setting a resistance constant connected to a line extending from a clock power source 15, and the resistor 16 is connected to one of input terminals of the Exclusive OR gate 17. Then, an end of the sensor 13 is connected to the line between the resistor 16 and the Exclusive-OR gate 17. The sensor 13 is composed of a capacitor 18 and a switch 19 connected in series, and the other end of the sensor 13 is grounded.
On the other hand a resistor 20 for setting a resistance constant os connected to another line extending from the clock power source 15, and the resistor 20 is connected to the other input terminal of the Exclusive-OR gate 17.
In this state, the capacitor 18 corresponds to a circuit constituted by the conductive rubber 10, the first electrodes 3 and the second electrode 4, while the switch 19 corresponds to a circuit constituted by the conductive rubber 10 and the carbon electrodes 6.
Incidentally, when the button 11 is depressed, the flexible leg 9 of the click rubber 7 is flexed so that the conductive rubber 10 of the click rubber 7 is brought into contact with the carbon electrodes 6 to establish electrical connection between the carbon electrodes. As the button 11 continues to be pushed down, the conductive rubber 10 is pushed against the first electrodes 3 and the second electrode 4 which is covered with the resist 5. According to the pressure contact, electrostatic capacitance in the conductive rubber 10 is varied so that output from the conductive rubber 10 is thereby changed.
In other words, after the switch 19 is turned on in the circuit 14, the output of the Exclusive-OR gate 17 is changed in accordance with the variation of the electrostatic capacitance of the capacitory 18.
FIG. 6 is a graph showing the relationship between pressure of the sensor provided by bringing the conductive rubber 10 into contact with the resist 5 and the output level therefrom. As shown by a solid line, the output level rises substantially in proportion to a rise of the pressure.
Howerver, in FIG. 6, there exists disersio0n in the output level for the pressure of the respective sensors 13 as shown by dotte lines. The dispersion may incur instability of the output level for the pressure of the sensor 13 so that reliabilty as the sensor 13 may be deteriorated.
It is considered that dispersion in a coating thickness of the resist 5 covering the first electrodes 3 and the second electrode 4 is one of the causes of the dispersion in the output level for the pressure. In fact, when the resist 5 having a coating thickness of 10 xcexcm are resist-printed on copper patters of the first electrodes 3 and second electrode 4, a coating thickness of the printed resist 5 will be 8 xcexcm to 16 xcexcm, and the dispersion in the output level for the pressure will occur.
It is therefore an object of the present invention to provide a pressure sensor in which the dispersion in the output level for the pressure of the pressure sensor is decreased.
In order to achieve the above object, according to the present invention, there is provided a pressure sensor comprising:
a base plate;
a first electrode, formed on the base plate;
a insulating laminate material, which coats the first electrode;
a conductive rubber; and
a click rubber, on which the conductive rubber is attached, for bringing the conductive rubber into a pressure contact with the first electrode coated with the insulating laminate material.
Preferably, a coating thickness of the insulating laminate material is substantially constant.
Preferably, the first electrode is formed into a C-shape in a plane view.
In this configurations, the dispersion in the output level for the pressure of the pressure sensor can be decreased.